Semiconductor light emitting devices comprising light emitting diodes (LEDs) are among the most efficient and robust light sources currently available. Illumination requires white color light sources, in particular white light sources of high color rendering properties. Various attempts have been made to make white light emitting illumination systems by using LEDs as radiation sources.
One method of obtaining white light is to use blue LEDs and convert part of the emitted light to yellow light (wavelength spectrum at about 580 nm) via wavelength converting materials, such as for example modified YAG:Ce based phosphors. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white.
Typically, this is done by arranging a phosphor-containing material, i.e. a wavelength converting material on the LED such that part of the light emitted by the LED is absorbed by the phosphors and is emitted as light of a wavelength different from that of the absorbed light.
However, one problem associated with such an arrangement is the color homogeneity of the light provided. The conversion strength of the wavelength converting material is controlled by the activator content (e.g. Ce as in YAG:Ce) and the path length of the blue light which travels through the ceramic element. This path is dependent on the thickness and scattering of the wavelength converting material. Typically, the ceramic element guides the blue light towards the edges. Since in this direction the path length is much longer than if the light emerges in a vertical direction, a higher conversion ratio is obtained and this results in the formation of a so called “yellow ring” when looking at larger angles.
In order to solve the problem of the formation of a yellow ring and to obtain a homogenous color impression, scattering must be introduced in the ceramic element.
WO 2006/097876 describes the use of a phosphor in a polycrystalline ceramic structure and a light-emitting element provided with the same comprising an LED in which a composite structure of phosphor particles is embedded in a matrix comprising a polycrystalline ceramic alumina.
WO 2006/097876 states that color homogeneity may be achieved by introducing pores and second phases that introduce light scattering. The porosity should not exceed 1% and the pore sizes should be kept small, e.g. smaller than 300 nm, preferably below 50 nm.
In ceramics, porosity is conventionally obtained by stopping sintering in an intermediate stage of the process in order to avoid the formation of a stable, fully densified ceramic body. Inherent to such a process, the control of a specific porosity level is difficult as a small temperature difference can cause large density (and porosity) variations having consequences for color conversion strength and thus color homogenity. Such ceramic elements are inappropriate for mass production.
Hence, there is a need in the art to provide a light emitting device which prevents the formation of a yellow ring around the device, and which allows for a more controlled and robust process to achieve the desired porosity during manufacturing, the device being easy and inexpensive to manufacture allowing for mass production.